1. Field of the Invention
The invention relates to an exhaust pipe, and in particular to an exhaust pipe of an internal combustion engine.
2. Description of Related Art
A catalyst for purifying (controlling) exhaust gas is provided in an exhaust passage of an internal combustion engine. The exhaust gas purification (control) function of such a catalyst can not be sufficiently used unless the temperature of the catalyst is equal to its activation temperature or higher. Thus, normally, the catalyst is warmed up using the heat of exhaust gas until the temperature of the catalyst reaches the activation temperature or higher. To accelerate the warming-up of the catalyst, the temperature of exhaust gas is increased by reducing the heat loss of an exhaust pipe. For example, Japanese Patent Application Publication No. 2003-286841 describes a technology in which a double pipe is used as an exhaust pipe to reduce the radiation of heat of exhaust gas flowing in the exhaust pipe and thereby increase the exhaust gas temperature.
According to the technology described in Japanese Patent Application Publication No. 2003-286841, however, there is a possibility that the catalyst may be excessively heated due to the exhaust gas heat when the exhaust gas temperature is sufficiently high after the catalyst has been warmed up, and this may cause degradation of the exhaust gas purification (control) performance of the catalyst.